Danfoss: CO2 booster system better than HFC

By R744.com team, Jan 07, 2009, 09:55 3 minute reading

A transcritical CO2 booster system, using only standard components, can be highly reliable and more efficient than a comparable R404a system and a R410a/CO2 cascade system. This has been proven by leading supplier Danfoss in field tests now summarised in a new article.

A transcritical CO2 booster system with gas bypass from the intermediate pressure receiver to the suction side of the high pressure compressors can outperform both an R410a/CO2 cascade system and an R404a-only system used in commercial refrigeration. This was found by Danish manufacturer Danfoss in laboratory as well as field tests over several months. The energy consumption of the cascade system was 99% of that of the R404a system, while the CO2 booster system showed the lowest energy consumption – approximately 96% of the R404a system.

Results have now been summarised in a new article published in December 2008.

System Design & Test Conditions

Danfoss in cooperation with partners designed a 10 kW (-30°C) and 24 kW (-10°C) refrigeration system for a small discount supermarket. After evaluating different systems of energy consumption and the availability of components, Danfoss decided for a CO2 booster system with a gas bypass from the intermediate pressure receiver to the suction side of the high pressure compressors. It was built with standard components available from normal suppliers of refrigeration equipment.

The system is divided into three pressure sections. The high pressure section begins at the high pressure compressor and continues through the gas cooler and suction line heat exchanger to the high pressure control valve. The design pressure in this section is usually between 90 and 120 bar. The intermediate pressure section begins at the high pressure expansion valve where the flow is divided into gas and liquid at the receiver. The gas phase is sent to the suction line of the high pressure compressors through a bypass valve. In a third section, the liquid flows to the expansion valves where it is expanded prior to continuing to the MT and LT evaporators. The gas from the LT evapo¬rator is compressed in the LT compressor mixing with the gas from the MT evaporator and forming the gas bypass. From here the gas enters the suction line heat exchanger and completes the circuit to the HP compressor. The design pressure is 40 bar for the MT section and 25 bar for the LT section.

The CO2 system, in operation since 1 March 2007, was then compared with a cascade system using R410a and CO2, and an R404a reference system. All systems were of similar cooling capacity and size, with a comparable load profile. The reference R404a system is a standard parallel system for both medium and low temperature. The cascade system uses a plate heat exchanger as condenser, with the low and medium temperature being served by two parallel systems with a common condensor. The systems in comparison had been running for one and two years respectively. All three systems were located in stores on Jutland, Denmark, which showed a similar turnover.

The Results

The CO2 transcritical booster system outperformed both HFC systems over a period of 37 weeks. Since its installation in March 2007, it has been running without major problems. Minor incidents concerning noise/vibrations and oil handling were solved and around 50 more such systems were installed during the first year.

Although Danfoss admits that the data available does not allow for a general conclusion that a transcritical system uses less energy than conventional systems, it confirms that this is definitely the case for the CO2 booster system tested. Its findings are backed up by other studies conducted by other manufacturers as well.


By R744.com team (@r744)

Jan 07, 2009, 09:55

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